1. Field of the Invention
The present invention relates in general to method and apparatus for displaying video images. In particular, the present invention relates to method and apparatus for displaying multiple fonts of video images in non-DMA (direct memory access) mode. More particularly, the present invention relates to method and apparatus for displaying multiple fonts of video images for television-game machines in non-DMA mode. Technical Background
Video images displayed by television-game machines are typically generated in two portions: a relatively static background image portion and an active subject image portion. The two portions are generated separately but are combined together to constitute a complete video image. They are generated separately due to their distinct characteristics. For example, the background image portion generally has a larger screen area, and therefore, it requires a larger data size than the subject image portion, but its pixels change less often as the video images advance. On the other hand, the subject image portion occupies a relatively smaller portion of the entire display screen, and thus, it requires less data. However, since the subject image portion displays the "subject" of a video story, it is generally characterized by a relatively larger video change which is reflected in the changes to the data of the displayed pixels.
In general, a conventional image display device processes the two portions of a video image in a time-shared manner. The background image portion is processed during scanning, and another processor in the device predicts the changes of the subject image portion that will appear in the up-coming scanning cycle. The color information of the predicted subject image portion is retrieved and stored in a register, which is then retrieved and combined with the color information of the background image portion at the time when the next scanning cycle commences. The combined color information is then supplied to a television interface circuit which generates television compatible display signals.
In such a conventional image display device, image pixel bit mapping schemes are employed for the subject image portion based on the efficiencies of image storage, retrieval and processing. An image pixel bit mapping scheme is considered efficient for the processing of the subject image portion of the video because of its smaller data size and more drastic data changes. However, when the subject image portion of the video becomes relatively large, in other words, the subject image portion occupies a relatively large portion of the display screen and therefore requires a larger image data size, such image pixel bit mapping scheme would require a larger and contiguous memory space allocation, which deteriorates the operating efficiency of the display memory and renders the memory management more difficult.